A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC/DC converters are frequently employed to advantage. DC/DC converters generally include an inverter, an input/output isolation transformer and a rectifier on a secondary side of the isolation transformer. The inverter generally includes a switching device, such as a field effect transistor (“FET”), that converts the DC input voltage to an AC voltage. The input/output isolation transformer, then, transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the converter. Conventionally, the rectifier includes a plurality of rectifying diodes that conduct the load current only when forward-biased in response to the input waveform to the rectifier.
Unfortunately, rectifying diodes suffer from a reverse recovery condition when there is a transition from a conduction stage to a non-conduction stage. During the reverse recovery condition, the current through the diodes reverse direction, causing excess energy to be stored in the leakage inductance of an isolation transformer and power to be lost in the diodes. The energy stored in the leakage inductance is dissipated in a resonant manner with the junction capacitance of the rectifying diode, causing oscillation (or ringing) and overshoot in the voltage waveform of the rectifying diode. As a result, the converter suffers efficiency losses that impair the overall performance of the converter. Therefore, efforts to minimize the losses associated with the rectifier and, more specifically, with the rectifying diodes will improve the overall performance of the converter.
A traditional manner to reduce the losses associated with the rectifying diodes is to introduce a snubber circuit coupled to the rectifying diodes. For instance, a resistor-capacitor-diode (RCD) snubber circuit is disclosed in “A 1 kW, 500 kHz Front-End Converter for a Distributed Power Supply System”, by L. H. Mweene et al., Proc. IEEE Applied Power Electronics Conf., p. 423–432 (1989), which is incorporated herein by reference. The RCD snubber circuit not only damps out oscillations in the rectifier's diode voltage, but also recovers a portion of the energy stored in the snubber capacitor to the output. During each switching transient, the reverse recovery energy due to the recovery process of the diodes is first stored in the snubber capacitor followed by a transfer of the energy to the output through the snubber resistor. During this process, some power is dissipated in the snubber resistor. As the output power increases, the power dissipated in the snubber resistor becomes significant thereby limiting the RCD snubber to lower power applications.
To reduce the power loss in the snubber resistor, a lossless snubber circuit is proposed in “High-Voltage, High-Power, ZVS, Full-Bridge PWM Converter Employing an Active Snubber” by J. A. Sabaste et al., 1991 VEPC Seminar Proc., pp. 125–130, which is incorporated herein by reference. This circuit operates in the same way as a RCD snubber circuit, except that the energy taken into the snubber circuit is recovered to an auxiliary inductor through the oscillation between the auxiliary inductor and the lossless snubber capacitor, after an auxiliary switch is turned on. However, the snubber circuit will lose its effectiveness if the converter is operated at very small duty ratio, resulting in insufficient time to discharge the energy stored in the snubber circuit.
An alternative approach is to employ a saturable reactor snubber circuit in series with the rectifying diode. The saturable reactor normally has an amorphous core that has the capability of being able to transition between low impedance (i.e., saturation) and high impedance with relatively low core losses. Therefore, when the rectifying diode is conducting, the reactor (in saturation) provides low impedance thereby allowing the current to flow freely. However, when the rectifying diode transitions from conduction to non-conduction stage and the reverse recovery condition occurs, the reactor provides a high impedance thereby reducing the reverse current flow. The saturable reactor has to be cooled by forced air, otherwise the reactor will run too hot. When applied to natural convection cooling power supplies, the temperature rise (e.g., up to 1200 Celsius) may not be acceptable.
Yet another alternative approach to manage the losses associated with the reverse recovery condition is to employ a clamp circuit coupled to the rectifying diodes as disclosed in “Snubber circuits: Theory, Design and Application”, by Philip C. Todd, Unitrode Power Supply Design Seminar Note, p. 2–1, 2–15 (1993), which is also incorporated herein by reference. The clamp circuit disclosed in Todd limits the peak voltage and reduces the stress across components within the converter. An advantage associated with such a circuit is that a clamp circuit does not dissipate energy in the converter.
Unfortunately, the clamp circuit introduced in Todd is generally limited to applications wherein the output voltage of the converter is fixed. Modifications can be made to the clamp circuit, however, to make it independent of the output voltage. For example, a coupling transformer may be connected across the main transformer effectively recovering excess transient energy to the primary side of the transformer. An impediment to the use of such a circuit is that the coupling transformer of the clamp circuit is generally comparable in size to the main transformer due to the large volts-second of the main transformer.
Accordingly, what is needed in the art is a circuit for a rectifier that suppresses a peak voltage associated with, for instance, the reverse recovery condition to thereby reduce the power losses associated with the rectifier.